Father Hewko clearly articulates that it's possible, from a theological standpoint, that a valid Sacrament could confer grace, but that the position is dangerous. One could make the same case about the Orthodox Mass, that, hey, it's valid, so I can go there to receive the Sacramets and receive grace. Why not, eh?
Bishop Williamson's answer, as Father Hewko rightly points out, will be taken by the faithful as a green light to assist at the NOM in the interests of receiving the graces of the Sacrament.
Father clearly makes the distinction that Johnson has always missed on this issue, namely, that while the Sacrament objectively has the "potential" to confer grace, whether it actually confers grace is dependent upon other factors, including the disposition of the recipient, and the context in which it is received. That's precisely what Father Hewko is correctly articulating here. In fact, various saints and theologians have also stated that the holiness and the dispositions of the priest can effect the degree to which the graces of the Mass are applied to the faithful. If the Mass is offered in an irreverent manner, the degree to which God will confer grace upon the souls present would likely be greatly diminished.
OBJECTIVELY speaking, the amount of grace available from a single Holy Communion is infinite, and reception of Holy Communion could and should transform each recipient immediately into a saint. But the gace that each recipient receives is "throttled" by extrinsic considerations, including the dispositions of the recipient, and even the reverence with which it the Mass (in which it was consecrated) has been offered, etc. There's no reason that God, being displeased with the sacriledgeous Prot Rite, with the Catholic Offertory replaced by a тαℓмυdic blasphemy, a Mass that Our Lord calls odious and containing words from the abyss (cf. Julie Marie Jahenny), there's no reason that God could not throttle the graces received to zero. If a Catholic were to receive Holy Communion in an Orthodox church, even if, say, done in good faith (let's say an ignorance Catholic thinks it's an Eastern Rite Catholic church), there's nothing to prevent God from not conferring grace to the soul from the Sacrament given in that context ... apart from perhaps what He might grant for the subjective dispositions alone, i.e. similar to what one might receive from a spriitual communion.